This invention relates to a high bandwidth optical fiber of high data-carrying capacity and a method for forming the same.
Optical fibers employed for lightwave communication are well-known in the art. The many advantages afforded by the use of light waves for signal transmission purposes over electrical waveforms of energy are also well-known and have led to increasing substitution of optical systems for electrical systems.
As background for the provided invention the following will be noted.
It is well-known that light entering an optical fiber does not propagate randomly as a continuum but rather is channeled into discrete modes having unique wave patterns.
If the optical fiber has an index of refraction which varies radially from a maximum at the core center to a minimum at the cladding interface, in accordance with an exponent .alpha. defining a roughly parabolic slope of the gradient, the bandwidth or information carrying capacity thereof is greatly increased. The latter fiber is known as a graded index fiber.
In graded index fibers, incident light rays apparently do not propagate in modes following sharp zig-zag paths but define substantially sinusoidal paths for propagating along the fiber length; oblique rays or modes in such fibers similarly follow helical paths. In any event, graded index fibers serve to minimize the effects of modal dispersion in light waves traversing the fiber. The degree to which dispersion is minimized, with resulting bandwidth enhancement, is dependent upon the index of refraction profile possessed by the specific fiber. In general, graded index fibers have an intrinsic superiority over step index fibers in information carrying capacity, e.g., 200 times the capacity, as the gradient of the index change has a constant effect of refocusing the light, i.e., reducing the dispersion.
Marcuse in his paper "Multimode Fiber with Z-dependent .alpha.-Value" appearing in Vol. 18, No. 13 of Applied Optics, July 1, 1979 discloses the dependence of the rms pulse width response of a graded index multimode fiber on the index of refraction profile of such fiber. Also, this paper discloses an extremely pronounced minimum rms pulse width response at a specific .alpha. value. This paper further discloses that the optimum pulse width response is achievable for fibers with nonoptimum .alpha. values, if the fiber .alpha. varies slowly along the fiber length and deviates on an average by equal amounts to either side of its (constant) optimum value.
Weirholt in Vol. 15, No. 23 of Electronics Letters, Nov. 8, 1979, proposes a high bandwidth fiber geometry with a radially varying .alpha. profile for achieving an almost flat rms spectral reponse over a spectral region of several hundred nanometers. Formation of such model is suggested by rigorous control the formation of the .alpha. profile through appropriate doping procedures. Weirholt suggests an overcompensated or lower .alpha. in the central fiber region and an undercompensated or higher .alpha. in the peripheral region.
In neither Marcuse nor Weirholt is there suggestion of .alpha. optimization by a simple twisting technique during fiber drawdown.
In accordance with the invention herein provided, the data carrying capacity of a graded index fiber is increased by preform rotation in the course of fiber drawing. As a result of such rotation the density of the drawn fiber core is progressively reduced radially of the fiber with an attendant substantially precise lowering in refractive index. Thus, by controlled preform rotation, the index gradient exponent .alpha. of a graded index fiber may be altered to approach or attain the optimum .alpha., thereby enhancing the refocusing effect of the fiber and hence its bandwidth for greater data carrying capacity. It is believed that the shearing action effected by relative twisting action between the preform and fiber in the plastic neck-down preform region stretches the inter-atom bonds such as silicon-oxygen bonds, which then are frozen in during the rapid fiber cooling phase of drawing, thereby progressively lowering the refractive index radially of the drawn fiber.
As this invention provides fibers with rotation induced lowering of the refractive index, the method disclosed must of necessity lower the .alpha. of graded index preforms, and therefore is useful with preforms which have an .alpha. higher than the final desired .alpha.. Thus, preform index of refraction gradients may be lowered to an optimum during fiber drawing by the rotation-induced lowering of the drawn fiber refractive index as will hereinafter be apparent.
The prior art has previously expended considerable effort in endeavoring to effect light mode velocity equalization by causing mode conversion, as in Miller et al. U.S. Pat. No. 3,687,514. As a result, the light energy of the various modes present spend roughly equal amounts of time in different modal configurations and arrive at the fiber end more nearly after the same average time. The Miller et al. patent deals with a step index fiber formed with geometrical irregularities by moving or "jiggling" a preform in the course of fiber drawing for accomplishing the desired mode conversion results. Miller et al. do not describe a change in core refractive index.
Marcatili U.S. Pat. No. 3,666,348 discloses preform control during fiber drawing to reduce unguided mode formation and light loss in a step index fiber.
Kaiser U.S. Pat. No. 3,969,016 discloses mode conversion by heated roller engagement with drawn fibers, relying on fiber axis shifts in the step index fibers processed.
Other patent art dealing with mode conversion in step index fibers comprises Presby U.S. Pat. No. 3,912,478 in which heat is employed for imparting geometrical changes for mode-conversion purposes.
Marcuse U.S. Pat. No. 3,909,110 discloses the building of core index fluctuations into the preform by employing modified doping procedures.
Olshansky U.S. Pat. No. 4,163,601 discloses the formation of perturbations in graded index fibers for mode conversion purposes by employing noncircular draw wheels.
In the foregoing, geometrical changes are imparted to the light waveguides for mode conversion purposes. There is no suggestion of the use of rotation to lower the refractive index of a fiber for purposes of attaining a desired change in the .alpha. or index of refraction profile.